1. Field of the Invention
The invention relates to a mobile communication system comprising at least mobile stations; base station systems, each comprising a base station controller and several base stations; mobile services switching centers, each comprising call control means and switch means for controlling and switching calls of mobile stations; and transcoder units, each comprising encoding and decoding means for decoding an encoded speech signal received from a mobile station, and for encoding a speech signal to be transmitted to a mobile station. For switching the calls, there are network connections that are allocated call-specifically between a base station, a base station controller and a mobile services switching centre, a speech signal being transmitted in transmission frames over said network connections, the base stations and the transcoder units comprising means for building, debuilding and synchronizing the transmission frames, the uplink transmission frames transmitted by the base station to the transcoder unit containing synchronizing information on the basis of which the transcoder unit times the transmission of the downlink transmission frames to the base station.
2. Description of Related Art
In cellular or trunked radio systems, generally referred to as mobile communication systems, a mobile radio station, i.e. a mobile station communicates with a fixed network via a fixed radio station, i.e. a base station that is located in a cell. Mobile stations may freely move from one cell to another in the area of the system. A general principle of the mobile communication networks is always to direct telephone communication to the area in which a mobile station is currently located. In order to be able to route the incoming calls to the mobile station, the radio network maintains location registers, in which the location of the mobile station is known with an accuracy of one or more base stations. A mobile services switching center or a base station controller control switching of a call to the correct base station. A base station controller is used when the base stations are grouped into base station systems comprising several base stations having a common base station controller. A mobile services switching center controls telephone communication within its service area and connects the base station controllers to the outside world.
GSM (Global System for Mobile Communication) is a pan-European telecommunication system, which is becoming a world-wide standard. FIG. 1 presents, very briefly a few basic components of the GSM system. A mobile services switching center (MSC) is in charge of switching incoming and outgoing calls. It performs similar operations as in the exchange of a public switched telephone network. In addition, it also performs operations typical only of mobile telecommunication, such as subscriber location management. Mobile radio stations, i.e. mobile stations are switched to the center MSC by means of base station systems. A base station system comprises a base station controller BSC and base stations BTS. A base station controller BSC is used for controlling several base stations.
The GSM system is completely digital, and speech and data transmission also take place completely digitally, which enables a uniform high quality of speech. From the point of view of the network, the most limited resource is the radio frequency link needed on the radio path between the mobile stations and the base stations. In order to reduce the radio frequency band needed by the radio connection, speech encoding is employed in the speech transmission. The speech encoding is more efficient than the 64 bit/s transmission typically employed in telephone networks. A mobile station must naturally comprise a speech encoder and decoder for speech encoding. In the network, various speech encoding and rate adapting operations are concentrated in a transcoder unit TRCU (Transcoder/Rate Adaptor Unit). The TRCU may be located in several alternative locations within the system, according to choices made by the manufacturer. When the transcoder unit TRCU is located remote from the base station BTS, information is transmitted between the base station BTS and the transcoder unit TRCU in so called TRAU frames. Four types of frames are defined, depending on the type of information included in them. These are speech frame, operation and maintenance frame, data frame, and a so called idle- speech frame.
A transcoder unit is typically located at the mobile services switching center MSC, but it may also be a part of the base station controller BSC or the base station BTS. A transcoder unit located remote from the base station BTS must know some radio parameters for effective decoding. In addition, the timing of the transcoder unit must be adapted for the transmission of the frames over the radio path so that the frames arrive from the transcoder unit at the base station synchronized with the transmission over the radio path (this minimizes the delay owing to a remotely located transcoder unit). Specific in band-signalling is employed for this control and synchronization of the transcoder unit on a 16 bit/s channel between the base station and the transcoder unit. The channel is used for the transmission of speech or data. The remote control of the transcoder unit is defined in recommendation GSM 08.60. In order to carry out synchronization, the first two octets of each frame contain 16 synchronizing bits. In addition, the first bit of the 16-bit words the frame is composed of (2 octets) is a check bit for synchronizing. All frames contain, in addition to bits including actual speech, data or operation/maintenance information, control bits for transmitting information on the type of the frame and a varying amount of other frame-type specific information. In addition, e.g. in speech and idle frames the last four bits are allocated for the above mentioned timing adjustment.
The interfaces of a transcoder unit are a 64 kbit/s PCM (Pulse Code Modulation) interface (A Interface) towards the mobile services switching center (MSC) and a 16 kbit/s GSM interface towards the base station BTS. In the context of these interfaces, terms uplink and downlink direction are used in the GSM recommendations. The uplink direction refers to the direction from the base station BTS to the mobile services switching center MSC. The downlink direction is opposite to the uplink direction.
When a call is established from a mobile station MS to a public switched telephone network (PSTN) in a system in accordance with the recommendations, the call-related signalling is transferred from the base station BTS to the mobile services switching center MSC, which, in turn, establishes connections between the connecting line to the public switched telephone network PSTN and the channel in the above mentioned A interface. Simultaneously, the transcoder unit TRCU is allocated and switched to the channel in the A interface. The mobile services switching center MSC further gives the base station controller BSC a command to connect the base station BTS with which the calling mobile station MS has a radio connection to the above allocated channel of the A interface. The base station controller BSC establishes a connection between the above allocated A interface channel and further the base station BTS with which the calling MS is communicating. The base station BTS is independently in charge of establishing the connections over the radio path. Thus, a connection is achieved in which a mobile station MS, a base station BTS, a base station controller BSC, a transcoder unit TRCU, a mobile services switching center MSC and a public switched telephone network PSTN are connected in series. In this case, encoded speech is transmitted via this connection between MS-TRCU and TRAU frames between BTS-TRCU.
When a call is going on between two mobile stations MS located within the area of the same base station or the same base station controller, the call is switched as to the calling mobile station MS in the same way as described above, but the mobile services switching center now establishes a connection between the A interface channel allocated for the calling mobile station MS and the A interface channel of the called mobile station MS. From this second transcoder unit, in turn, a connection is established back to the same base station controller BSC, and further to the base station of the called mobile station MS. In other words, all calls between mobile stations MS subject to e.g. the same base station BTS are routed, such that the mobile services switching center, and each call is allocated two transcoder units. Long routing of this kind wastes the transmission capacity between the base station controller BSC and the mobile services switching center MSC, as well as the switching capacity of the MSC. Using or renting transmission links may have a crucial effect on the expenses of the network operator. Secondly, a solution of this kind consumes transcoder resources, since two transcoder units are switched along with each MS-MS connection. The number of the transcoder units must be planned in accordance with traffic peaks, in which case the number of the transcoder units is too high in a normal situation, both as to the costs and the number of the units. Furthermore, successive transcoding and decoding measures between the base station controller BSC and the mobile services switching center MSC impair the quality of the speech signal and cause delay of the effective signal.
The above mentioned facts have not caused too much inconvenience so far, since relatively few calls have been mobile-to-mobile calls.
In the future, however, cordless telephone exchanges, "radio PABX", will be used subject to mobile communication systems to replace internal telephone exchanges in offices. In private telephone exchanges of this kind, large numbers of calls between extensions, or so called internal calls are made, and thus a large number of calls on the level of the mobile communication system will be switched from MS to MS, as well. Thus, normal call switching of the same type as in GSM, via a mobile services switching center, will cause inconvenience, since a very large number of calls take place within the area of one single base station controller BSC, or even in the area of the same base station BTS.